The invention relates to an axial-flow low-pressure steam turbines and to axial/radial multi-channel diffuser and waste steam housing for guiding the waste steam from the blades with few losses.
A diffuser of this type is described in DE 44 22 700. The diffuser disclosed in this document is provided after the last row of rotating blades of a low pressure steam turbine with an axial flow inlet and a radial flow outlet. The diffuser is designed with respect to optimized turbine performance by way of the greatest possible pressure recovery. For this purpose, the first partial pieces of the inner and outer diffuser ring each are oriented in relation to the hub or, respectively, the blade carrier, with an inflexion angle. This measure serves to homogenize the total pressure profile above the channel height of the diffuser in the area of the last row of rotating blades. The diffuser furthermore is provided with a radially outward curved guide plate that divides it into an inner and an outer channel. Hereby flow ribs impacted by the flow either radially or diagonally have been provided in the outer and inner channel. The guide plate is used both for deflecting and guiding the waste stream. The flow ribs have the purpose of supporting the guide plate and, in particular, reduce the spin in the delay zone, so that they also contribute to an optimization of the pressure recovery. However, realized flow ribs only are able to achieve optimum spin reduction with a specific operating load. At a different operating load, the spin reduction is not necessarily optimized. A diffuser with this kind of measure therefore only achieves optimum pressure recovery at a certain operating load. The flow ribs and their attachment to the guide plates furthermore are associated with relatively high construction expenditure. In addition, the supersonic gap flow interferes with the remaining subsonic flow.
EP 581 978, especially in FIG. 4 of this publication, discloses a multi-channel waste gas diffuser for an axial-flow gas turbine with axial flow inlet and radial flow outlet. This multi-channel diffuser is provided with three zones along its length. The first zone is constructed in the manner of a bell diffuser and extends as one channel from the last row of rotating blades to the outlet plane of several flow ribs. Here also, the diffuser rings are provided with inflexion angles that have been established so that the total pressure profile is homogenized. Downstream from the flow ribs, the second zone has flow-guiding guide rings that form several channels. The third zone is used for a major deflection of the waste gas flow in radial direction and then merges with the chimney of the gas turbine. For this purpose, the guide rings of the second zone are extended across the length of the third zone, whereby they are curved there. The second zone has a minor deflection yet high diffuser effect; the third zone a major deflection, yet has a very moderate diffuser effect.
It is the objective of the present invention to create, for a low-pressure steam turbine, an axial/radial multi-channel diffuser with waste steam housing that, in comparison to diffusers according to the state of the art, achieves an improved steam recovery, thus increasing the effectiveness of the low-pressure steam turbine. In addition, the multi-channel diffuser should be simultaneously optimized for as many operating conditions of the steam turbine as possible and should be associated with reduced construction expenditure. Finally, the waste steam housing should be adapted to the diffuser with respect to turbine performance.
This objective is realized with an axial/radial three-channel diffuser with an exhaust steam housing. The three-channel diffuser is provided with three partial diffusers, i.e., an inner, middle, and outer partial diffuser, which are formed by an inner diffuser ring, and outer diffuser ring, and two guide vanes provided between the diffuser rings. A first partial piece of the inner diffuser ring is hereby arranged in relation to the hub at an inflexion angle oriented inward, towards the rotor axis, and a first partial piece of the outer diffuser ring is arranged at an inflexion angle oriented outward in relation to the blade channel at the level of the last row of rotating blades, away from the rotor axis.
In the axial/radial three-channel diffuser according to the invention, in particular, the two guide plates extend across the entire length of the diffuser. They are unevenly distributed between the inner and outer diffuser ring, so that the distribution of the surface area over the three partial diffusers in the inlet surface area of the diffuser is uneven. In the inlet plane, the majority of the inlet surface area hereby is part of the inner and middle partial diffuser, and a small part of the inlet surface area is part of the outer partial diffuser. Furthermore, the starting tangents of the two guide plates, together with the limits of the blade channel on the hub side and housing side, which approximate each other in a straight line, form an at least approximately common intersection point above the end stage of the low-pressure steam turbine in the meridian plane. Finally, the guide plates are located as close as possible to the last row of rotating blades, whereby the distance between the last rotating blade row and the leading edges of the guide plates are determined by the minimum distance that is permissible for all operating conditions.
This describes the characteristics of the diffuser in its interaction zone with the last stage.
The diffusion zone of the diffuser is characterized by the following characteristics.
The ratio of the outlet surface area to the inlet surface area of the individual partial diffusers is greater than 2 for the middle partial diffuser and smaller than 2 for the outer partial diffuser. For the inner partial diffuser, the corresponding geometric surface area ratio ranges from 1.5 to 1.8.
Furthermore, for the middle partial diffuser, the ratio of its length to its channel height in the inlet surface area is at least equal to 4. For the outer partial diffuser, the ratio of length to channel height in the inlet surface area is at least equal to 10, and for the inner partial diffuser, the corresponding ratio is at least equal to 2.5. Based on these relatively high length to channel height ratios, the deflections of the partial diffusers are accordingly relatively small.
The ratio of the outlet surface area to the inlet surface area of the diffuser overall is approximately 2.
Finally, the waste steam housing of the diffuser is designed so that the size of the surface area of the dividing plane between the top and bottom half of the waste steam housing is adapted to the size of the outlet surface areas of the partial diffusers.
The two guide plates hereby are used to divide the diffuser channel into three partial diffusers in which the blade waste flow is guided. The resulting flow guidance is hereby the better, the more partial diffusers are present for the same overall diffuser. In contrast, the more guide plates are provided, the more friction losses and obstructions are created. The number chosen here, i.e., three partial diffusers and two guide plates, has the advantage that optimized flow guidance is achieved with justifiable friction losses at the guide plate surface areas and obstructions.
The guide plates and partial diffusers bring about a guidance and stabilization of the blade waste flow as well as a deflection into a radial direction. Since the guide plates extend over the entire length of the diffuser, this guidance is further supported.
The radial extension of the partial diffusers furthermore is used to naturally reduce the tangential speed. Because of this, the partial diffusers are optimized for all operating conditions with respect to a reduction of the tangential speed. The construction expenditure for the guide plates is also rather low, and the reduction of the tangential speed does not require any further constructive measures, such as deflection and flow ribs.
The flow guidance and stabilization is further brought about, in particular, by distributing the diffuser inlet surface area over the three partial diffusers. A majority of the inlet surface area is part of the inner and middle channel, so that the majority of the flow is guided from the blades to the waste steam housing. The smaller part of the inlet surface area is part of the outer channel, through which the supersonic gap flow as well as the flow from the turbine influenced by the gap flow is taken up and is deflected meridionally and is guided, shielded from the majority flow, to the waste steam housing. This shielding prevents flow interferences between the majority flow and the high-energy gap flow that would interfere with the diffuser effect.
The minimum distance between the last row of blades and the leading edges of the guide plates further promotes an optimal shielding of the gap flow and prevention of flow interferences and streamline convergences.
The ratio of length to channel height of each partial diffuser of 2.5 or more enables a gentle deflection from the axial or diagonal to the radial flow direction, which prevents separation of the delayed flow even at a ratio of outlet surface area to inlet surface area of 1.6.
The guidance and stabilization of the blade waste flow through the three partial diffusers, the shielding of the high-energy gap flow as well as the gentle deflection based on the length of the channels in relation to their channel heights overall achieve a homogenization and reduction of the total pressure profile at the level of the last row of rotating blades. The resulting added performance results in an increased efficiency of the low-pressure steam turbine.
The design of the diffuser according to the invention is based on a reverse design process, during which the existing flow fields are determined first. Then the respective ideal flow fields are calculated from this, and the geometry of the diffuser is determined based on these ideal flow fields. In particular, this three-channel diffuser has been designed at limit load conditions. At the limit load, a flow field, for which a three-channel diffuser with an orientation of the starting tangent of its guide plates according to the invention achieves the highest pressure recovery, was determined. It was established experimentally, that the geometry resulting from this design is superior to the state of the art diffusers over the entire operating range. This design furthermore has the advantage that a higher turbine performance is achieved with the same condenser pressure, or that the same turbine performance is achieved with a higher condenser pressure, so that a smaller, cheaper cooling system is required for the steam turbine.
Special embodiments of the invention below disclose additional, special characteristics of the interaction zone of the diffuser.
In a first, special embodiment of the invention, the starting tangents of the guide plates are in an angle range around the first inflexion point of the guide plates and around a reference starting tangent that extend at least approximately through the first inflexion point of the guide plate and through the inflexion point of the blade channel limits that approximate each other in a straight line.
In another special embodiment of the invention, the outer partial diffuser accounts for a part of the entire flow inlet surface area of the diffuser in the range from 10-12%. Of the remaining inlet surface area, 55-60% is distributed to the inner partial diffuser, 30-35% to the middle partial diffuser.
In another embodiment, the distance between the leading edges of the guide plates and the trailing edge of the last rotating blade accounts for 4% of the entire height of the rotating blade row.
In another embodiment, the leading edges of the guide plates are constructed with a profile at the flow inlet of the diffuser, resulting in a gentle acceleration at the inlet into the partial diffusers.
In additional embodiments, the diffusion zone of the diffuser is characterized as follows.
The guide plates each are carried by struts or supports extending from the inner and outer diffuser ring to the two guide plates. The middle partial diffuser remains free from any supports and therefore has minimal flow interference and losses.
In another, special construction of the waste steam zone of the diffuser, a waste steam metal plate is arranged in a radial extension at the end of the guide plate between the inner and outer partial diffuser. This waste steam guide plate achieves a better flow distribution in the waste steam housing, so that flow losses are minimized and the condenser is supplied more evenly.